[Amps] SB220 Meter blown

[Gary Schafer]

Jim is correct. It depends on the meter circuit that is used. You need to
calculate how much voltage will appear at the meter.
I have a drake L7 that I added protection diodes to. I was going to just
hang diodes across the meter shunt but in looking at it I found that it was
a very low value and that the meter did not have much series resistance.
Putting the diode directly across the meter or across the meter shunt would
allow too much current in the meter before the diodes would conduct.

I ended up lowering the series meter resistor and placing another resistor
in series where I placed the diodes. In case of a glitch this would allow
the voltage across the diodes to rise high enough to conduct before the
meter voltage gets to a dangerous level but not conduct at full amp
operating current.

73
Gary  K4FMX 

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim
> Sent: Friday, May 15, 2020 10:20 PM
> To: amps at contesting.com
> Subject: Re: [Amps] SB220 Meter blown
> 
> WRONG.
> 
> .6 or .7 or slightly more during an "event" volts is still 
> enough to blow up the meter.
> 
> 
> Thanks
> 73
> Jim W7RY
> 
> On 5/15/2020 4:08 PM, Adrian wrote:
> > from ; http://www.somis.org/sb-220ci.html
> >
> > "6. Meter damage can be avoided by connecting two, ordinary 
> = or >1A, 
> > any PIV, silicon rectifiers across the terminals on each meter. The 
> > two diode arrows should point in opposite directions."
> >
> >
> > On 16/5/20 1:10 am, Artek Manuals wrote:
> >> I am resurrecting a SB220 that had an apparent classic Grid/plate 
> >> short event. One of casualties appears to be the right hand meter 
> >> movement. This is the one for Grid Current and HV. I am pretty far 
> >> into it and the meter movement measures open , the internal shunt 
> >> resistor is blackened ( and open?)
> >>
> >> Anyone got a surplus movement from a parted out unit? 
> Nothing found 
> >> on evil-bay at the moment
> >>
> >> Dave NR1DX
> >>
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